Ethereum 2.0 Phase 1 -- Shard Data Chains
Notice: This document is a work-in-progress for researchers and implementers.
Table of contents
Introduction
This document describes the shard transition function (data layer only) and the shard fork choice rule as part of Phase 1 of Ethereum 2.0.
Custom types
| Name |
SSZ equivalent |
Description |
ShardSlot |
uint64 |
a shard slot number |
GweiDelta |
int64 |
a signed Gwei delta |
Configuration
Misc
| Name |
Value |
MIN_BLOCK_SIZE_PRICE |
2**0 (= 1) |
MAX_PERIOD_COMMITTEE_SIZE |
2**7 (= 128) |
SHARD_HEADER_SIZE |
2**9 (= 512) |
SHARD_BLOCK_SIZE_TARGET |
2**14 (= 16,384) |
SHARD_BLOCK_SIZE_LIMIT |
2**16 (= 65,536) |
Initial values
| Name |
Value |
SHARD_GENESIS_EPOCH |
TBD |
Time parameters
| Name |
Value |
Unit |
Duration |
SHARD_SLOTS_PER_EPOCH |
2**7 (= 128) |
shard slots |
6.4 minutes |
EPOCHS_PER_SHARD_PERIOD |
2**8 (= 256) |
epochs |
~27 hours |
State list lengths
| Name |
Value |
Unit |
HISTORY_ACCUMULATOR_VECTOR |
2**6 (= 64) |
state tree maximum depth |
Rewards and penalties
| Name |
Value |
SHARD_BLOCK_SIZE_PRICE_QUOTIENT |
2**3 (= 8) |
Signature domain types
| Name |
Value |
DOMAIN_SHARD_PROPOSER |
128 |
DOMAIN_SHARD_ATTESTER |
129 |
Containers
ShardBlock
class ShardBlock(FlatContainer):
shard: Shard
slot: ShardSlot
beacon_block_root: Hash
parent_root: Hash
state_root: Hash
block_size_sum: uint64
body: List[byte, SHARD_BLOCK_SIZE_LIMIT - SHARD_HEADER_SIZE]
attestation_bits: Bitvector[2 * MAX_PERIOD_COMMITTEE_SIZE]
attestation_signature: BLSSignature
signature: BLSSignature
class ShardBlockHeader(FlatContainer):
shard: Shard
slot: ShardSlot
beacon_block_root: Hash
parent_root: Hash
state_root: Hash
block_size_sum: uint64
body_root: List[byte, SHARD_BLOCK_SIZE_LIMIT - SHARD_HEADER_SIZE]
attestation_bits: Bitvector[2 * MAX_PERIOD_COMMITTEE_SIZE]
attestation_signature: BLSSignature
signature: BLSSignature
ShardState
class ShardState(FlatContainer):
shard: Shard
slot: ShardSlot
history_accumulator: Vector[Hash, HISTORY_ACCUMULATOR_VECTOR]
latest_block_header: ShardBlockHeader
block_size_sum: uint64
# Fees and rewards
block_body_price: Gwei
older_committee_deltas: Vector[GweiDelta, MAX_PERIOD_COMMITTEE_SIZE]
newer_committee_deltas: Vector[GweiDelta, MAX_PERIOD_COMMITTEE_SIZE]
ShardCheckpoint
class ShardCheckpoint(Container):
slot: ShardSlot
parent_root: Hash
Helper functions
Misc
compute_epoch_of_shard_slot
def compute_epoch_of_shard_slot(slot: ShardSlot) -> Epoch:
return Epoch(slot // SHARD_SLOTS_PER_EPOCH)
compute_shard_period_start_epoch
def compute_shard_period_start_epoch(epoch: Epoch, lookback: uint64) -> Epoch:
return Epoch(epoch - (epoch % EPOCHS_PER_SHARD_PERIOD) - lookback * EPOCHS_PER_SHARD_PERIOD)
Beacon state accessors
get_period_committee
def get_period_committee(state: BeaconState, shard: Shard, epoch: Epoch) -> Sequence[ValidatorIndex]:
active_validator_indices = get_active_validator_indices(state, epoch)
seed = get_seed(state, epoch)
return compute_committee(active_validator_indices, seed, shard, SHARD_COUNT)[:MAX_PERIOD_COMMITTEE_SIZE]
get_shard_committee
def get_shard_committee(state: BeaconState, shard: Shard, epoch: Epoch) -> Sequence[ValidatorIndex]:
older_committee = get_period_committee(state, shard, compute_shard_period_start_epoch(epoch, 2))
newer_committee = get_period_committee(state, shard, compute_shard_period_start_epoch(epoch, 1))
# Every epoch cycle out validators from the older committee and cycle in validators from the newer committee
older_subcommittee = [i for i in older_committee if i % EPOCHS_PER_SHARD_PERIOD > epoch % EPOCHS_PER_SHARD_PERIOD]
newer_subcommittee = [i for i in newer_committee if i % EPOCHS_PER_SHARD_PERIOD <= epoch % EPOCHS_PER_SHARD_PERIOD]
return older_subcommittee + newer_subcommittee
get_shard_proposer_index
def get_shard_proposer_index(state: BeaconState, shard: Shard, slot: ShardSlot) -> ValidatorIndex:
epoch = get_current_epoch(state)
shard_committee = get_shard_committee(state, shard, epoch)
active_indices = [i for i in shard_committee if is_active_validator(state.validators[i], epoch)]
seed = hash(get_seed(state, epoch) + int_to_bytes(slot, length=8) + int_to_bytes(shard, length=8))
compute_proposer_index(state, active_indices, seed)
Shard state mutators
process_delta
def process_delta(state: BeaconState, shard_state: ShardState, index: ValidatorIndex, delta: GweiDelta) -> None:
epoch = compute_epoch_of_shard_slot(state.slot)
older_committee = get_period_committee(state, shard_state.shard, compute_shard_period_start_epoch(epoch, 2))
newer_committee = get_period_committee(state, shard_state.shard, compute_shard_period_start_epoch(epoch, 1))
if index in older_committee:
shard_state.older_committee_deltas[older_committee.index(index)] += delta
elif index in newer_committee:
shard_state.newer_committee_deltas[newer_committee.index(index)] += delta
Genesis
get_genesis_shard_state
def get_genesis_shard_state(state: BeaconState, shard: Shard) -> ShardState:
return ShardState(
shard=shard,
slot=ShardSlot(SHARD_GENESIS_EPOCH * SHARD_SLOTS_PER_EPOCH),
block_body_price=MIN_BLOCK_SIZE_PRICE,
)
get_genesis_shard_block
def get_genesis_shard_block(state: BeaconState, shard: Shard) -> ShardBlock:
return ShardBlock(
shard=shard,
slot=ShardSlot(SHARD_GENESIS_EPOCH * SHARD_SLOTS_PER_EPOCH),
state_root=hash_tree_root(get_genesis_shard_state(state, shard)),
)
Shard state transition function
def shard_state_transition(state: BeaconState,
shard_state: ShardState,
block: ShardBlock,
validate_state_root: bool=False) -> ShardState:
# Process slots (including those with no blocks) since block
process_shard_slots(state, shard_state, block.slot)
# Process block
process_shard_block(state, shard_state, block)
# Validate state root (`validate_state_root == True` in production)
if validate_state_root:
assert block.state_root == hash_tree_root(shard_state)
# Return post-state
return shard_state
def process_shard_slots(state: BeaconState, shard_state: ShardState, slot: ShardSlot) -> None:
assert shard_state.slot <= slot
while shard_state.slot < slot:
process_shard_slot(state, shard_state)
# Process period on the start slot of the next period
if (shard_state.slot + 1) % (SHARD_SLOTS_PER_EPOCH * EPOCHS_PER_SHARD_PERIOD) == 0:
process_shard_period(state, shard_state)
shard_state.slot += ShardSlot(1)
def process_shard_slot(state: BeaconState, shard_state: ShardState) -> None:
# Cache state root
previous_state_root = hash_tree_root(state)
if state.latest_block_header.state_root == Bytes32():
state.latest_block_header.state_root = previous_state_root
# Cache state root in history accumulator
depth = 0
while state.slot % 2**depth == 0 and depth < HISTORY_ACCUMULATOR_VECTOR:
state.history_accumulator[depth] = previous_state_root
depth += 1
Period processing
def process_shard_period(state: BeaconState, shard_state: ShardState) -> None:
# Rotate rewards and fees
state.older_committee_deltas = state.newer_committee_deltas
state.newer_committee_deltas = [GweiDelta(0) for _ in range(MAX_PERIOD_COMMITTEE_SIZE)]
Block processing
def process_shard_block(state: BeaconState, shard_state: ShardState, block: ShardBlock) -> None:
process_shard_block_header(state, shard_state, block)
process_shard_attestations(state, shard_state, block)
process_shard_block_body_fee(state, shard_state, block)
def process_shard_block_header(state: BeaconState, shard_state: ShardState, block: ShardBlock) -> None:
# Verify that the slots match
assert block.slot == state.slot
# Verify that the beacon chain root matches
parent_epoch = compute_epoch_of_shard_slot(state.latest_block_header.slot)
assert block.beacon_block_root == get_block_root(state, parent_epoch)
# Verify that the parent matches
assert block.parent_root == hash_tree_root(state.latest_block_header)
# Save current block as the new latest block
state.latest_block_header = ShardBlockHeader(
shard=block.shard,
slot=block.slot,
beacon_block_root=block.beacon_block_root,
parent_root=block.parent_root,
# `state_root` is zeroed and overwritten in the next `process_shard_slot` call
block_size_sum=block.block_size_sum,
body_root=hash_tree_root(block.body),
attestation_bits=block.attestation_bits,
attestation_signature=block.attestation_signature,
# `signature` is zeroed
)
# Verify proposer signature
proposer_index = get_shard_proposer_index(state, state.shard, block.slot)
pubkey = state.validators[proposer_index].pubkey
domain = get_domain(state, DOMAIN_SHARD_PROPOSER, compute_epoch_of_shard_slot(block.slot))
assert bls_verify(pubkey, hash_tree_root(block.block), block.proposer, domain)
# Verify body size is a multiple of the header size
assert len(block.body) % SHARD_HEADER_SIZE == 0
# Verify the sum of the block sizes since genesis
state.block_size_sum += SHARD_HEADER_SIZE + len(block.body)
assert block.block_size_sum == state.block_size_sum
Attestations
def process_shard_attestations(state: BeaconState, shard_state: ShardState, block: ShardBlock) -> None:
pubkeys = []
attestation_count = 0
shard_committee = get_shard_committee(state, state.shard, block.slot)
for i, validator_index in enumerate(shard_committee):
if block.attestation_bits[i]:
pubkeys.append(state.validators[validator_index].pubkey)
process_delta(state, shard_state, validator_index, get_base_reward(state, validator_index))
attestation_count += 1
# Verify there are no extraneous bits set beyond the shard committee
for i in range(len(shard_committee), 2 * MAX_PERIOD_COMMITTEE_SIZE):
assert block.attestation_bits[i] == 0b0
# Verify attester aggregate signature
domain = get_domain(state, DOMAIN_SHARD_ATTESTER, compute_epoch_of_shard_slot(block.slot))
message = hash_tree_root(ShardCheckpoint(shard_state.slot, block.parent_root))
assert bls_verify(bls_aggregate_pubkeys(pubkeys), message, block.attestation_signature, domain)
# Proposer micro-reward
proposer_index = get_shard_proposer_index(state, state.shard, block.slot)
reward = attestation_count * get_base_reward(state, proposer_index) // PROPOSER_REWARD_QUOTIENT
process_delta(state, shard_state, proposer_index, reward)
Block body fee
def process_shard_block_body_fee(state: BeaconState, shard_state: ShardState, block: ShardBlock) -> None:
# Apply proposer block body fee
proposer_index = get_shard_proposer_index(state, state.shard, block.slot)
block_body_fee = state.block_body_price * len(block.body) // SHARD_BLOCK_SIZE_LIMIT
process_delta(state, shard_state, proposer_index, -block_body_fee) # Burn
process_delta(state, shard_state, proposer_index, block_body_fee // PROPOSER_REWARD_QUOTIENT) # Reward
# Calculate new block body price
block_size = SHARD_HEADER_SIZE + len(block.body)
QUOTIENT = SHARD_BLOCK_SIZE_LIMIT * SHARD_BLOCK_SIZE_PRICE_QUOTIENT
price_delta = GweiDelta(state.block_body_price * (block_size - SHARD_BLOCK_SIZE_TARGET) // QUOTIENT)
if price_delta > 0:
# The maximum gas price caps the amount burnt on gas fees within a period
MAX_BLOCK_SIZE_PRICE = MAX_EFFECTIVE_BALANCE // EPOCHS_PER_SHARD_PERIOD // SHARD_SLOTS_PER_EPOCH
state.block_body_price = Gwei(min(MAX_BLOCK_SIZE_PRICE, state.block_body_price + price_delta))
else:
state.block_body_price = Gwei(max(MIN_BLOCK_SIZE_PRICE, state.block_body_price + price_delta))
Shard fork choice rule
The fork choice rule for any shard is LMD GHOST using the shard attestations of the shard committee and the beacon chain attestations of the crosslink committee currently assigned to that shard, but instead of being rooted in the genesis it is rooted in the block referenced in the most recent accepted crosslink (i.e. state.crosslinks[shard].shard_block_root). Only blocks whose beacon_block_root is the block in the main beacon chain at the specified slot should be considered. (If the beacon chain skips a slot, then the block at that slot is considered to be the block in the beacon chain at the highest slot lower than that slot.)